The present invention relates to an additive for adding to a melt for producing spheroidal graphite cast iron, a production method of spheroidal graphite cast iron and articles of spheroidal graphite cast iron. More specifically, the present invention relates to an additive effective for producing a spheroidal graphite cast iron having a large graphite spheroid count (the number of spheroidal graphite particles) with little chilled structure and a production method of such a spheroidal graphite cast iron.
The inoculation treatment of a melt for producing spheroidal graphite cast iron has been done generally with a view to increasing the mechanical strength, improving the metal structure, preventing the formation of chilled structure, improving the mass effect, preventing the formation of shrinkage cavity, etc. As the inoculant, alloys mainly comprising a graphitizing Si, such as Fe--Si(75% ) alloy, etc., have been widely used in the art and added to the melt just before the casting operation. As the spheroidizing elements (nodulizing elements), Mg, Ce, Ca, Ba, Sr, Y, etc. have been known and used in combination with the inoculant.
The inoculant for promoting the graphitization includes 75% Fe/Si (for example, 76.5% Si, 0.5% Ca, 1.3% Al), Ca/Si (for example, 57.5% Si, 14.0% Ca, 1.0% Al), etc., and is characterized by containing Si and Ca as the main component and other components such as Al, Sr, Mg, Ti, Zr, Ce, Ba and RE (rare earth elements) in the form of metal or alloying component.
Regarding the mechanism of graphitization promoted by inoculation, various opinions have been proposed. One of them explains that a heterogeneous nucleus formed by oxides, sulfides or carbides contributes as the substrate for the graphitization. Another explains that the melt is locally supersaturated with carbon during Si is diffused throughout the melt just after the inoculation, and nuclei for graphitization are formed in the supersaturated portion. The sulfide theory is based on the fact that Ca and Al in inoculant are easily converted into sulfides and graphite grows on the sulfides. However, there are many phenomena that cannot be explained by the sulfide theory. Therefore, it has not yet been established which theory is correct.
As mentioned above, various theories have been proposed on the promotion of forming graphite spheroids by inoculation. Although, the mechanism of the promotion has not yet been established, the addition of the inoculant are widely practiced in the art to prevent the formation of chilled structure, to improve the matrix, to minimize the shrinkage, and many proposals have been presented on the inoculant, other additive alloys and production method of spheroidal graphite cast iron.
Japanese Patent Laid-Open No. 54-115612 discloses an additive for desulphurizing a molten iron and molten steel and spheroidizing graphite. The additive is produced by uniformly mixing 10-50% of particles and small pieces of magnesium or an alloy containing 80% or more of magnesium, each being subjected to surface coating treatment with an organic and inorganic compound and having a size of 10 mm or less, 30-80% of at least one selected from the group consisting of magnesia, silicon carbide, zirconium oxide and alumina, 5-40% of at least one selected from the group consisting of charcoal, activated carbon and coke, and 1-10% of bastnaesite and/or witherite (barium carbonate); adding a binder to the mixture; granulating; and shaping. On page 2, lines 6-14 of lower left column, it is taught that "in the present invention, a mixture mainly containing particles and small pieces of magnesium or an alloy containing 80% or more of magnesium and magnesia, silicon carbide, etc. is added with charcoal, coke, bastnaesite and witherite (barium carbonate), and then granulated or shaped by using various binders. One of the most important features of the invention is to protect magnesium from substances such as moisture, air, etc., which are reactive to magnesium, by subjecting the surface of magnesium and magnesium alloy to inactivation coating treatment."
Japanese Patent Laid-Open No. 54-33818 relates to a method of producing spheroidal graphite cast iron characterized in that a shaped article of a mixture containing an ultrafine powder of metallic magnesium and an oxide of magnesium is added to a molten metal. The shaped article is produced by shaping a mixture containing metallic magnesium and magnesium oxide, and heating the mixture. Alternatively, the shaped article is produced by shaping a mixture containing metallic magnesium and magnesium oxide with an organic high-molecular weight compound. It is taught that metallic magnesium obtained by reducing magnesium oxide with carbonaceous substance is particularly preferable. The mixture comprises 5-90 weight % of metallic magnesium and 95-10 weight % of magnesium oxide. Metallic magnesium is in the form of ultrafine powder (0.01-10 .mu.m), and the surface thereof is coated by magnesium oxide. The coating of magnesium oxide is taught to effectively avoid violent reaction of metallic magnesium at the time of addition to a molten metal and secure a gentle reaction. Further, the ultrafine metallic magnesium proceeds the reaction in the molten metal uniform. It is also taught that the shaped article of the mixture containing ultrafine metallic magnesium and oxide of magnesium is effective when added in an amount of 0.05 weight % or more in terms of magnesium based on the amount of molten metal. By such an addition amount, graphite is effectively spheroidized by magnesium oxide to finely disperse the graphite particles in the cast ion, and the number of the graphite particles per unit rupture cross-section can be increased.
Japanese Patent Laid-Open No. 54-124814 discloses a metallurgical additive comprising magnesium and magnesium oxide which is prepared by heating metallic magnesium and/or a magnesium alloy to generate a vapor of magnesium, contacting the vapor with a gas containing carbon monoxide, and cooling. The resultant mixture containing Mg, MgO and carbon formed by the reduction of CO may be used with or without shaping using an organic binder or by heating. The weight ratio is 5-90 parts for Mg and 95-10 parts for MgO. It is taught that MgO prevents the violent reaction of Mg in the molten metal and the fine Mg particles generate ultrafine bubbles of Mg vapor in the molten metal which disperse through the molten metal. Therefore, the reaction efficiently proceeds, and the desulphurization rate and the efficiency of Mg utilization are extremely increased. Also, since the surface of Mg and MgO is covered with an organic binder, the additive is easy to handle and can be stored over a long period of time.
In the production of spheroidal graphite cast iron, it is important to finely disperse a great number of graphite spheroids (nodules) in the melt in view of preventing the formation of chilled structure by controlling the cooling rate of the solidified melt and improving the various mechanical properties of the cast product. Therefore, many consideration has been given to the improvement of the inoculant such as Si-based alloy, the nodulizing material, the inoculation method such as the stream inoculation, etc.
However, since the nodulization (spheroidization) and the inoculation effect are insufficient in the production methods conventionally employed in the art, it is difficult to produce a good spheroidized graphite structure as well as to obtain a spheroidal graphite cast iron having good properties due to an excess amount of chilled structure and an insufficient ferritization caused by an insufficient graphitization and a decreased number of graphite spheroids. In particular, in the production of a spheroidal graphite cast iron having both a thin section with a thickness of 5 mm or less and a thick section with a thickness exceeding 5 mm in as cast state, the thin section is likely to form the chilled structure due to rapid cooling of the solidified melt and a post-heat treatment is frequently required to change the pearlitic matrix to the ferritic matrix. Since the solidification of the melt slowly proceeds in the thick section, the graphitization is insufficient in the thick section.